In an inkjet head formed by arranging a plurality of nozzles for ejecting ink droplets in one direction, the volume of ink droplets ejected from the nozzle is not always uniform. Thus, density unevenness occasionally occurs even if the same number of ink droplets is ejected from each nozzle to print a solid image. In a case in which printing is carried out in a printing area wider than a width of a nozzle arrangement direction of the inkjet head, the printing area is divided in a width direction and a plurality of the inkjet heads is arranged for each divided area to carry out the printing in some cases. In such a case, a level difference in density occurs at a boundary between the heads.
The main reason why the volume of the ink droplets ejected from each nozzle is not uniform is the occurrence of dispersion in the configuration of the inkjet head. For example, the diameter of each nozzle or the volume of a pressure chamber communicating with each nozzle is not necessarily certain. Such dispersion in the configuration is always caused by characteristics of a processing machine used at the time of manufacturing the inkjet head.
Conventionally, there is a technology which adjusts an ejection amount of the ink droplet for every nozzle by correcting a pulse width of a drive pulse signal applied to each actuator respectively corresponding to each nozzle. With the technology, it is possible to make uniform an amount of the ink droplet ejected from each nozzle. However, in order to make uniform the amount, correction data for correcting the pulse width for each nozzle must be calculated. For example, 320 pieces of the correction data must be calculated for an inkjet head which has 320 nozzles, which takes too much time and labor.